Pulp products, such as paper and board are manufactured commercially on large scale machines, such as fourdrinier, double wire and cylinder machines. In the manufacturing process, pulp stock, containing fibres such as wood pulp, rag or recycled paper fibres, along with suitable additives such as fillers, retention aids, pigments, flocculating agents, defoaming agents or binders in large volumes of water, are mixed to a slurry.
Fourdrinier, double wire and cylinder machines are equipped with different screens in the forming section of the machine. The fourdrinier machine has an endless moving fourdrinier wire screen, double wire machines have two endless wire screens and the cylinder machines have cylindrical revolving screens.
In a fourdrinier machine, the slurry of pulp stock is fed from a head box onto the continuously moving fourdrinier wire in the forming section of the machine. As the slurry advances on the wire down the forming section of the machine an initial amount of the water in the pulp stock drains through the wire into drainage units to form a web of pulp product supported on the wire. Close to the downstream end of the wire additional amounts of water are forcibly removed from the web by means of suction boxes in contact with the lower surface of the wire.
Once the web of pulp product reaches the end of the wire it is passed over a suction couch roll which extracts further water from the web of pulp product which emerges from the couch roll as a self supporting web of pulp product which can be peeled off the wire and subjected to further processing steps, such as pressing, drying and calendering to remove water and improve the surface and finish of the product.
For some applications, in particular for printing applications, such as light weight coated and machine finished coated paper it is desirable to apply a coating to the pulp product. Such coating is traditionally applied to both surfaces of the dry paper sheet in a size press, which generally consists of a pair of rolls with loading capability, forming a nip, through which the sheet passes. The coating material is applied to the surface of the rolls by a dip roll or spray pipes so that the ingoing nip is flooded with the coating material, thereby impregnating both surfaces of the sheet. Coating materials include starch sizing, filler, pigment and binder.
The conventional size press has a number of disadvantages and size press operation reduces overall machine efficiency. Size presses have a large number of moving parts that wear and require adjustment and they are labour intensive in terms of both general maintenance and time in equipment clean-up.
There are many machines operating today, such as groundwood type machines for newsprint, which do not include size presses because, traditionally, the paper run on these machines was not coated. It is desirable in some cases to add coating capability to these machines in response to changing demands in the paper industry for different types and grades of paper.
It can be both time consuming and costly to undertake the extensive alterations required to add conventional coating systems. In most cases, to apply a uniform surface coating would require the addition of a size press, roll coater, etc. This type of coating applicator would require more space than is available on typical newsprint type machines. Drying capacity would have to be increased because of additional moisture added to the pulp product at the size press. In most cases, this would require increasing the length of the machine which in turn, would see the calender, reel and, possibly, the winder relocated.
Sprays have been used to impregnate modifying agents and additives substantially throughout the thickness of a forming paper web (U.S. Pat. No. 2,112,540 to McAndrews). Spray systems have very few moving parts and require little maintenance. Such sprays are generally located upstream from the suction boxes so that the modifying agent or additive is dispersed throughout the wet web (U.S. Pat. No. 2,112,540 to McAndrews and No. 3,287,207 to Treat). Dispersing agents have been used to facilitate incorporation of the additive into the web, U.S. Pat. No. 3,560,334 to Arledter.
In order to eliminate the problems of coating dried pulp products, attempts have been made to apply coating materials to the web of pulp product in the forming section of the machine before it passes over the suction couch roll and is removed from the forming section as a self supporting web of pulp product and is fed into the presses and dryers. Processes used to spray fillers and additives are described, for example, in U.S. Pat. No. 2,373,914 to Quinn.
There are a number of problems associated with the use of spray systems for applying coating materials to a web of pulp product in a machine and it has not been possible to obtain an even, complete coverage at machine speeds.
Traditionally, in the pulp and paper industry coating material is sprayed by pressure type nozzles which employ the fluid pressure to disperse the fluid, creating large droplets of liquid, resulting in spotty coverage of the web. Typical spray systems used in the industry propel the fluid at a high velocity, generating sufficient force to cause a ricochet effect when the fluid impacts on the web resulting in a spotty uneven finish. With typical high pressure application, the centre of the stream is more concentrated causing streaks on the coated surface while the outer edges of the spray fan are lost to the atmosphere, with a typical transfer efficiency of less than 50% The outer edges of the fan may also dry before reaching the substrate, contributing to the poor transfer efficiency. The poor transfer efficiency may also contribute to equipment contamination as overspray is carried in the air and may be deposited on any surface that it may come into contact with, thereby contaminating the equipment and work environment.
Accordingly, there is a need for a simple, flexible and efficient method for applying a film of coating material to a web of pulp product on a machine.